This invention relates generally to footwear and particularly to apparatus and methods for keeping a person's feet warm when the ambient temperature is considerably less than normal human body temperature. Still more particularly, this invention relates to an insole for use inside boots, shoes or the like for holding a heat source to provide warmth to the wearer's feet.
Various attempts have been made to keep a person's feet warm in cold environments. Improved insulation and waterproof footwear are helpful in preventing cold feet. However, many individuals still experience discomfort and risk of injury from cold feet in outdoor activities during cold weather because insulation only retains body heat without providing any source of heat.
One attempt to provide a heat source in a boot or the like involves placing an electric heating element in the boot liner and using a battery to supply energy to the heating element. Another attempt to solve the problem of cold feet has involved the use of electrical heating elements in socks. Foot warming devices that use electric heating elements are expensive and inconvenient to use. The batteries for such devices are expensive and require long recharging times to be effective. Battery packs are bulky, add unnecessary weight for the user to carry and may interfere with the user's movements. A battery has the characteristic that as the ambient temperature decreases, the battery's power output decreases. Therefore, as the need for heat increases, the battery powered systems decrease in effectiveness.
German Pat. No. 3,544,856 to Walter discloses an insole that incorporates a heat dispensing material in a pocket formed in the insole padding. U.S. Pat. No. 4,658,515, issued Apr. 21, 1987 to Oatman discloses a laminated insole having pockets filled with an insulating material. The outer surfaces are formed of a heat reflective material. U.S. Pat. No. 4,249,391, issued Feb. 10, 1981 to Yoshida discloses footwear suitable for use as a house slipper having porous bags containing an exothermic agent. Yoshida discloses the use of small holes in the porous bags to facilitate heat flow from to a wearer's foot.
The prior art heated insole devices are unsuitable for long term wear in a shoe, boot or the like. The problem of cold feet for people engaging in outdoor activities in cold weather remains unsolved by the prior art.